The invention relates to an arrangement for an axial bearing in a drilling machine comprising a frame; a percussion device fitted in the frame; a shank positioned on an axial extension of the percussion device; means for rotating the shank; and an axial bearing fitted in the frame to receive axial forces acting on the frame through the shank, the axial bearing being formed by several pistons accommodated in axial housings formed in the frame along a periphery circumscribing the shank and interconnected with a conduit system, the pistons being further arranged to act on the shank so as to force it towards the forward portion of the drilling machine under the influence of a hydraulic fluid acting on the back surface of the pistons.
In hydraulic percussion drilling machines presently in use, the percussion device fitted in the frame is intended to apply successive axial impacts on a shank attached to a drill rod. The shank is mounted rotatably and axially slideably to the frame e.g. by a frame bushing which is in engagement with a rotation mechanism supported by the frame. The frame in turn is secured to a feed carriage on which the drilling machine is displaced along the feed rail of the drilling equipment.
On drilling a rock, an impact impulse is reflected from the rock to the drilling machine, and the force caused by the impulse has to be received in the drilling machine somehow. For this purpose, various flexible axial bearing arrangements have been developed for drilling machines to protect them against reflected impact-like stress impulses. Such arrangements include those disclosed in FI Patent Specification 58816, DE Auslegeschrift 2 738 956, SE Published Specification 440 873 and DE Offenlegungsschrift 2 610 619.
These flexible axial bearing arrangements known from the prior art have the drawback of being complicated, in addition to which they require a great number of seals and enable no adjustment of flexibility, that is, the rigidity of the axial bearings is invariable. A further drawback is that the flexing phenomenon occurs with a delay and depends on the feed power applied to the drilling machine.
FI Patent Application 861851 discloses an arrangement in which the axial bearing comprises several pistons positioned radially around the shank, whereby the pressure of a hydraulic fluid acts on one end of the pistons so that the pistons adjust the position of the shank within a predetermined area. In certain cases, however, it is necessary that the percussion point of the shank can be determined precisely in each particular case while maintaining the flexibility of the axial bearing, which cannot be fully accomplished with this arrangement.
The object of the present invention is to provide an arrangement for an axial bearing in a drilling machine which avoids the drawbacks of the prior art described above. This is achieved by means of an arrangement of the invention, which is characterized in that the length of travel of some of the pistons towards the forward portion of the drilling machine is limited so that when they are in their foremost position and the shank is supported by the pistons, the percussion surface of the shank is positioned substantially at its optimal percussion point, whereby the pressure of the hydraulic fluid acting on the back surface of the pistons at least during the drilling is arranged to be such that the total force exerted on the shank by all the pistons so as to force it forwards exceeds the feed force acting on the drilling machine during drilling.
An advantage of the arrangement of the invention is that when the pressure of the hydraulic fluid causes the pistons to travel forwards, some of the pistons stop in their foremost position, so that the shank is always positioned at its optimal percussion point when it is supported by the pistons. After the impact, however, some of the pistons are able to follow the shank during the percussion movement so that they deaden the return movement of the shank before it reaches the percussion point during the return impulse. When the shank reaches the percussion point, all the pistons deaden the return impulse efficiently. Whenever a new impact is to be made, the shank is always at the percussion point, since the force acting on the pistons is altogether greater than the feed force, whereas the feed force exceeds the total force produced by the pistons capable of following the shank after the movement of the pistons having their stroke length limited to the percussion point has stopped. A further advantage of the invention is that it is simple to manufacture and the pistons of different stroke length are very simple to construct and hence economical to manufacture.